Particulate emissions from an engine can be reduced with a particulate filter trap and a regeneration system to periodically clean the filter trap of particulates by incineration. Generally, durable and acceptable filters for particulate traps have been developed by the art which have included wire mesh (see U.S. Pat. No. 3,499,269) and, more advantageously, rigid ceramics preferably in a monolithic honeycomb cellular wall structure (see U.S. Pat. Nos. 4,276,071; 4,329,162; and 4,340,403).
Systems of regeneration, developed by the state of the art using filters may now be categorized as: (a) electrical heating, (b) hydrocarbon fuel fed burners, (c) catalyst impregnated traps, and (d) engine throttling. Regeneration systems (b) through (d) have undesirable aspects associated therewith. With fuel fed burners, complex hardware is required, which affects reliability and safety (see U.S. Pat. No. 4,167,852 and Japanese Pat. No. 55-19934). With known catalyst impregnated traps, there is increased sulphate formation, less than desirable durability, generation of unnecessary unburned hydrocarbons and CO, and hardware complexity. With engine throttling, regeneration cannot be carried at low vehicle speeds and light loads (see U.S. Pat. Nos. 4,211,675 and 3,499,269).
Regeneration by electrical heating is advantageous because it offers reliability and simplicity. It can be carried out under any vehicle operating condition, with a minimum increase in fuel economy without affecting emissions content at the outlet of the automotive exhaust system (see U.S. Pat. Nos. 4,270,936; 4,276,066; and 4,319,896). However, the electrical heating regeneration systems to date have not proven successful because they have required an inordinate amount of electrical energy to raise the temperature of the particulates to a level where ignition takes place, such requirement being beyond the on-board capabilities of the vehicle.
What is needed is a regeneration system for a filter trap that (a) is electrically powered but requires a much reduced on-board power supply, and (b) achieves quicker, more efficient regeneration of a particulate laden trap.